LEDs based on InAlGaN multiple quantum wells (MQWs) generally suffer from two problems. The first is the so called “Quantum Confined Stark Effect” (QCSE), which is due to internal electric fields caused by spontaneous and piezoelectric polarization. Specifically, this internal electric field distort the MQWs and as a result the overlap of the electron and hole wave functions is reduced, leading to reduced internal quantum efficiency (IQE). The second prominent problem is the efficiency reduction at high injection current density, which is known as “efficiency droop”.
Although LEDs have enjoyed commercial success in the display backlighting market, a major barrier for LEDs to enter the general lighting market is the problem of efficiency droop. Compared with traditional lighting products such as incandescent or florescent light bulbs, white LEDs have higher energy efficiency in converting electrical power input to optical power output. However, currently this is only the case when the LEDs operate at relatively low current densities. At high current density, the efficiency of LEDs is reduced, and the optical output power eventually saturates and decreases. To further reduce the cost per lumen, the ability to obtain high optical output power from single LED devices is needed.
Similarly, in the case of UV-LEDs, to enter large markets such as industrial curing and water purification, having a single LED that operates with high efficiency at high injection current conditions is also critical. To date, commercial UV-LEDs emitting in the UV-C range required for disinfection (<300 nm) can only operate at below 30 mA, where the output power from a single device is limited to a few hundred microwatts. Also, the radiative efficiency of UV-LEDs reduces drastically when they are operated at high current densities, where most of the input electrical energy is lost in device heating, eventually leading to device failure.
To date, multiple mechanisms have been proposed to explain the efficiency droop, including carrier delocalization, auger recombination and carrier overflow. While all of these mechanisms offer reasonable physics-based explanations for efficiency droop, no consensus on which one is the most plausible cause has been reached yet. Nevertheless, direct correlation can be established between efficiency droop and current density (to be more specific, the carrier density in quantum wells of the active region). In other words, it is only at high current densities when the quantum wells are populated with excess carriers (electrons in the conduction band and holes in the valence band) that the efficiency starts to decrease. Therefore, reducing the current density and/or reducing the carrier overflow in an LED device, while at the same time achieving high output power, are natural solutions to efficiency droop.
A number of methods have been employed to reduce the current density in LEDs. The current density at which an LED operates is defined as the injection current (measured in Amperes) divided by the area of the LED die (measured by centimeter squared). A simple solution to reduce the current density of the LED device, and therefore reduce the carrier density of the active region, is to increase the area of the LED chip (i.e. chip size). Large-area chip LEDs (usually defined as chip area larger than 1 mm2) can endure higher drive current compared to small chip size (smaller than 1 mm2), and therefore can produce higher optical output power. However, increasing the chip size will inevitably reduce the number of LEDs that can be produced from an LED wafer, which increases significantly the cost of LED production. Furthermore, in case of UV-LEDs, the resistivity of the n-type AlGaN layer limits the current spreading length of such devices, and therefore only the peripheries of the large-size chip will produce light from electron-hole recombination while the center area of the LED will not produce light. Because of these drawbacks in both cost and performance, simply using large size chip is not considered a viable solution to efficiency droop, particularly in UV-LEDs.
Another solution that has been adopted recently in the blue LED domain is to increase the thickness of the well layer and therefore reduce the carrier overflow in the quantum well in the active region. However, such thick quantum wells suffer from strong electron-hole wave function separation, due to strong QCSE induced by built-in polarity in nitride materials, and therefore reduce the IQE of such devices. Recently, there have been considerable new developments in producing thick quantum wells on non-polar or semi-polar substrates, and therefore removing or reducing the spontaneous polarization and the QCSE. However, though non-polar and semi-polar nitride epitaxy technology has achieved great success recently in the InGaN material system for blue-violet LED with reduced efficiency droop, in the AlGaN material system for ultraviolet LEDs the non-polar and semi-polar technologies are still under development. Current AlGaN epilayers grown on non-polar or semi-polar substrates suffer from high defect density and are difficult to dope efficiently. In addition, to maximize the injection efficiency in UV-LEDs, one needs to rely on polarization doping techniques in order to overcome the large ionization energies of n- and p-type dopants in wide bandgap semiconductors.
Alternatively, instead of using thick quantum well layers in the active region, one can also increase the number of quantum wells in the active region, where the carriers are distributed in a number of quantum wells. However, while such a method may seem viable in the case of blue-violet LEDs, it is not applicable for reduction of efficiency droop in UV-LEDs. The holes in AlGaN material have very high effective mass, and small mobility and diffusion length. Therefore, the holes cannot easily be transported from one well to another after populating the quantum well adjacent to the electron blocking layer. Therefore, increasing the number of quantum wells in UV-LEDs will lead to uneven carrier distribution across the wells, and electron-hole separation in different quantum wells, and therefore will reduce the IQE of UV-LEDs.
Thus, there remains a need to improve the efficiency of visible and UV LEDs, particularly at high injection current density.